I you are an adolescent male (ie a teenager) and you have a choice between Pfizer and Moderna - I suggest you ask for Pfizer.
Data shows the risk of myocarditis/pericarditis is much higher (in adolescent males) from Moderna than it is from Pfizer (about 50% more cases from Moderna than Pfizer here in Australia). The risk to females is also a little higher - but significantly less than males for some reason.
Apparently the risk of myocarditis/pericarditis from catching COVID is even worse - so vaccines are still very much recommended (even if you can only get Moderna), but you can mitigate some of the risk if you have the ability to choose which vaccine you get.
To quantify the risk, the rate of likely myocarditis following a 2nd dose of Pfizer in 12-17 yo males is 11.8 per 100,000 doses, while Moderna is 16.8 per 100,000 doses. Most cases of myocarditis are mild and cases usually resolve after a few days with treatment and rest, but some do require hospitalisation - with most patients who are admitted to hospital here in Australia discharged within 4 days.
More data here from the TGA (Australia's government department who monitors these things):
COVID-19 vaccine weekly safety report - 16-12-2021
I should also point out that our vaccination rates in Australia are now some of the highest in the world - despite our slow start. We have 81.2% of 12-15 year olds having had at least their 1st dose and 88.7% of 16-19 year olds with at least their 1st dose - most of them have already had their 2nd dose too (74.7% and 83.3% respectively).
It's worth noting that here in Australia, booster vaccines are not yet approved for under 18s.
My son is now 13 years old and has had 2 doses of Pfizer without any major issues (just the usual sore arm).
I'm due for my booster in a couple of weeks.